Measurement of acoustical properties of materials



May 14, 1940., w D, MQUNCE 2,200,476

MEASUREMENT OF ACOUSTICAL PROPERTIES 0F MATERIALS Filed DSC. 5l, 1935 4 Sheets-Sheet l Y Y Osc/LA rol REC 972362 /0 z J7 May 14 1940- w. D. MouNcE 2,200,476

MEASUREMENT 0F ACOUSTICAL PROPERTIES 0F MATERIALS Filed DSC. 51, 1935 4 Sheets-Sheet 2 .MPL/Fl 7/7 Osc/1.174 roze @j 4 Sheets-Sheet I5 W. D. MOUNCE Filed Dec. 51, 1935- (//72/ /z ,E /y

MEASUREMENT 0F ACOUSTICAL PROPERTIES 0F MATERIALS AMPL /F/Ea @EC-ORDER Y W/Z//m/ 3/// Q //U\4 m\\/ /f///////m// w\/\////c//,///// ////\\\M\\ m, \K////(////////\\\ May 14, 1946.

May 14 1940- w.l|: MouNcE 2.200.476

MEASUREMENT 0F ACOUSTICAL PROPERTIES OF MATERIALS all' dit"

Patented May 1'4, 194e Stand @il Development poration oi Delaware This invention relates to the measurement oi acoustical properties of media; more particularly it relates to the measurement of acoustical impedance, attenuation, and of velocities of propagation of elastic waves in the media. I

It is an object of this invention to utilize for geophysical exploration the acoustical properties of the earth which, in the sense here employed, are dependent on the density, elasticity, and viscosity of the materials comprising the earth.

More speclcally the method can be used for measuring the velocity, acoustic attenuation, and acoustic impedance of elastic waves in the earths strata penetrated by drill holes; from the results of such measurements in drill holes, it is possible to deduce geologic sections through the drill holes. also, the methods can be used for determining the acoustical impedance and attenuation or', or the velocity in the ground by mounting the apparatus on a truck'. and driving the truck ,along the surface ci the ground; in this way information is derived concerning the variation of acoustic properties in direrent geologic strata at the surface of the earth. Other applications oi the method will suggest themselves to those familiar with the art.

Fig. l is a vertical sectional view through the earth showing the application or the apparatus to a bore hole.

Fig. 2 is a vertical sectional view through the earth showing dlagrammatically a preferred form oi apparatus for measuring the velocity of elastic waves'ln the earth.

Fig. 3 is a vertical sectional view through the earth showing diagrammatically a preferred form of apparatus for measuring the acoustical attenuation of the earth in which the sending and receiving stations .are spaced from each other.

Fig, d is a vertical sectional View through the earth showing diagrammatlcally an alternative form of the apparatus for measuring the acoustical impedance oi the earth using a. single instrument.

Referring particularly to Figs. 1 and 2 of the drawings, reference numeral I designates a. bore hole in the earth which penetrates earth strata 2, 3, t and the like. A. cable I is suspended in the hole and carries a generator of elastic waves t. a receiver t is suspended by a cable It from the generator t. In the embodiment illustrated, the container of receiver t carries also auxiliary electrical apparatus I2. Elastic waves generated in the generator 9 pass through the surrounding medium in the well which usually will be a drills. ufr or acens s or .t D. Mounce, Houston, Tex., lgnor to a cor- L 193%, Serial No. 56,963

ing mud IS. Alternatively the medium may be water, air or the like and some of the advantages of the invention` will be retained. The elastic waves' pass from the generator 9 through the drilling mud Iii and through the adjoining earth stratum 3 as illustrated by lines 25, 2t, 2l and 2t in Fig. 1. In passing through the stratum 3, the acoustical properties of the stratum alter the energy received at receiving station 8. The elastic energy received at 8 is converted to electrical energy by the auxiliary electrical apparatus it, and is transmitted to the surface of the earth through the cables I3 and l to a recording device Il. Variation in the vertical elevation oi the generator and receiving apparatus are aected by means of a reel it or the like, over which the cable l passes. The recording apparatus may be a recording galvanometer Il. A suitable oscillator It for actuating the electrical apparatus may be disposed at the surface of the earth as illustrated. Preferably the elastic wave generator d and the receiver il are spaced from each `other longitudinally in the hole a uniform predetermined distance. 'I'he cable I3 contains electrical conductors, and is insulated from generator 9 and receiver 8 by means of a material such as rubber composition, hemp rope or the like oi a lower velocity and a higher attenuation to acoustic energy than the surrounding medium of the earth.

Referring particularly to Fig. 2 a preferred form of apparatus is illustrated for measuring the velocity of elastic Waves through the strata. In the figure, numeral El designates the medium. which is to be` measured, while numeral 2 designates a geologic stratum lying `above medium 3, both media being penetrated by the bore hole i. As shown here, the apparatus comprising the generator 9v, the detector or receiver 8, and the auxiliary apparatus I2 is lowered down the bore hole I. This generator consists of a stiff daphragm El which is displaced by an electromagnet t, and actuated periodically and momentarily by an electrically operated switch` 32, which is controlled by oscillator I8 at the surface of the earth through conductors I9 and 2li. These conductors are contained in the cable I of Fig. 1. Numeral 33 designates a battery for driving the diaphragm. The auxiliary apparatus I2 is as follows. In series with the driving magnet t is a potentiometer 3d, the voltage drop of which applies a positive grid potential to the gas trlode 35 when a current pulse flows through the magnet 6. The control grid of vacuum tube 36 is connected to the receiver or detector B through the ampliiier I0, and is negatively biased by the battery 31 so that its plate current does not ow until a positive voltage pulse is applied to its grid by the detector 8 through the amplier I0. The detector 8 comprises a diaphragm 2| which is displaced by the elastic energy; the diaphragm being connected to a pivotally mounted core 22 through a link 23 elects displacement of the core. The core and an inductance coil 24 are in the eld of a magnet 29 so that a positive voltage pulse is created proportional to the elastic energy received by the diaphragm. When current is ilowing through gas triode 35 and resistance 3|, condenser 44 is charged to the potential of battery 45 minus the potential drop in gas triode 35, point 6I being charged positively and point 60 negatively. Now when detector 8 causes a positive pulse to be applied to the grid of gas triode 36, plate' current begins to ow in this tube, causing a positive potential from condenser 443 to be applied momentarily to the cathode-of gas triode 35, which stops its plate current. At this time no current is flowing in potentiometer 36, consequently gas triode 35 remains nonconducting until another positive pulse is applied to its grid. When gasvtriode 36 is conducting, current ows through resistance 43, and condenser 44 is charged with point 60 positive and point 6I negative. .At thispoint a positive pulse applied to the grid of gas triode 35 reverses the above behavior. In other words, the two gas triodes 35 and 36 are so connected that when platel current is flowing' in one oi them, this current may be stopped by applying a positive voltage pulse to the grid of the other; or, at any time, plate current flows in one and only one of the gas triodes 35 and 36.

At the beginning of operation, plate current is liowing in gas triode 36. Now when switch 32 is closed momentarily (it obviously must be open at the time the impulse reaches the receiver) diaphram 5 is displaced by electromagnet 6, and plate current ows in gas triode 35 and through resistance 4I, which extinguishes gas triode 36 at the same time the sound impulse from the diaphragm is sent out. When plate current ows in gas triode 35, and resistance 4|, condenser 38 begins to charge through resistance 40. When the rst energy of the elastic impulse from the diaphragm reaches the detector 8, such vas through a path 25, the electric energy generated is amplified by I0 and applies a positive pulse to the grid of gas triode 36 which causes plate current to start ilowing in gas triode 36 and resistance 43, extinguishing 35, thereby removing the charging voltage of condenser 38. 'I'he voltage of condenser 38 is measured with the vacuum tube voltmeter tube 39 and ammeter I1. Gas triode 35, resistance 40, and condenser 38 are so chosen that for the time that plate current flows in gas triode 35, the voltage across condenser 38 is only a small part of the voltage across resistance 4I; hence the reading of meter I1 is determined by the time required by the sound impulse from generator 9 to travel to the detector 8. Since the distance between 9 and 8 is fixed, the meter I1 may be calibrated in units of vvelocity of the medium between 9 and 8. Battery 53 supplies lament current for tubes 35, 36, 39 and 50. Battery 42 furnishes the necessary grid bias for gas triode 35. B battery 45 supplies the plate voltages for the two gas triodes 35 and 36, respectively, and for the vacuum tube voltmeter 39. Current readings which are inversely proportional to the velocity are obtained from the meter I1. 41, 48 and 5| are large resistances, which prevent the grid currents in these three tubes from becoming excessive when their grids are positive.

there i`s no current flowing in resistance 43, but whose plate resistance drops to a low value when its grid becomes positive, discharging condenser 38 in a shorter time than was required to charge it through resistance 40. This is desirable when, instead of a single pulse as described above, a periodic series of impulses is used. In this case the vacuum tube voltmeter 39 with ammeter I1 and proper grid bias 49 gives a reading .of the mean voltage across condenser I8.

' It is evident that rthe lower the velocity of sound in the medium 3, the longerthe time required for the elastic pulses to travel from generator 9 to receiver 8 and, therefore,` the longer the time during which the condenser 38 is being charged, hence the greater the current indicated by the ammeter I1. On the other hand, if the velocity of sound in medium 3 is high, the condenser 38 is permitted to charge for only a short time, so that the current indicated by the ammeter I1 is small. Small currents in ammeter I1 are an indication of high velocities, while large currents indicate low velocities, hence the ammeter I1 may be calibrated in units of velocity.

1f the medium between generator 9 and receiver 8 is a complex one, comprising several possible paths by which the impulse travels from generator 9 to receiver 8, such as earth 3, drilling mud I5 and connecting cable I3, the interval of time between impulses from the generator must be greater than the time required for any appreciable amount of energy to travel from generator 9 to receiver 8 by any of the possible paths, in order that the time which condenser 38 is charging is the smallest time required for an appreciable amount of energy of an impulse to travel from generator 9 to receiver 8. Thus, the velocity indicated by ammeter I1 is that of the highest velocity, or shortest time of travel-path,

between generator 9 and receiver 8.

When the device is used along the surface of the ground the impulses travel from diaphragm 5 to detector 8 through the air andalso through the ground. In cases where the velocity of sound through the ground is less than through the air, the device will measure the velocity of sound in air. When used in a bore hole lled with water or drilling mud, the velocity of sound through the mud is about 5000 feet per second and the velocity through the surrounding medium is generally mulch higher, so that in this case the velocity of sound through the surrounding medium will be determined.

Fig. 3 shows a preferred embodiment for the measurement of the acoustical attenuation. between two points of a medium. In the figure, generator 9 is driven by oscillator 14, Whose frequency is kept constant for any single measurement but may be changed for diierent measurements of the same stratum. Receiver 8 is located at a point in the drill hole removed from the generator, similarly to that illustrated in Fig. 1. The electrical energy supplied by the oscillator 14 is measured by ammeter 11. An amplier I0 is connected between receiver 8 and the amplitude recorder I1. In this embodiment, energy from the generator 9 is transmitted through medium 3, the acoustical attenuation of which determines the amplitude measured by I1.

50 is a tube with negative grid bias i 52, whose plate resistance is very high when If the oscillator 14 supplies energy at a constant u rate, .the recorded amplitude is determined by the acoustical attenuation oi the medium between the detector 8 and generator t. I! the distance between generator 9 and the detector 8 5 is kept constant, the relative, or absolute, acoustic attenuation of various media may be read directly by recorder il. When the device is used in a bore hole, filled with drilling iiuid, the elastic energy from the generator 9 is conducted to the l receiver d both through the drilling uid and through the surrounding earth. However, since the uid in the well is of the same composition throughout the depth of the well, comparative readings taken at diiierent points in the well will l indicate changes in the elastic properties of the earth strata. at these points.

In Fig. 4 is shown apparatus for measuring the apparent4 acoustic impedance Vof a medium from a. single point. Numeral 3 designates the medium of which it is desired to measure the acoustical impedance. Numerals 2 and. d designate other media abutting on medium d. An oscillator 'M' is used to drive a diaphragm b by means of an electro-magnet t, the driving current being indicated by the ammeter Tl connected in the circuit. The mechanical energy from the diaphragm is transmitted either directly to the material to be measured, or thiough an inter-medium such as air, water or mud l5. Attached to d@ the diaphragm is a device 8, which may consist of a coil caused to vibrate in the iield of a magnet by the diaphragm, the device being connected to the amplitude recorder il. The acoustical impedance of the medium surrounding the 'il diaphragm determines the ratio of the amplitude of diaphragm E as read `by meter il to the amount of electrical energy furnished by the oscillator it through meter 'il'. If the oscillator be made to supply energy at a. constant rate, the ed amplitude measuring device il may be calibrated in units of acoustical impedance; this having been done, the apparent acoustical impedance of any medium adjacent to the diaphragm may be read directly, td Various changes and alternate arrangements may be made within the scope of the appended claims, in which it is my intention to claim all novelty inherent in the art as broadly as the prior art permits.

I claim: l. Apparatus for measuring the acoustical attenuation of elastic waves through a given medium, which comprises means for generating elas tic waves in the medium, means for measuring y the energy supplied to actuate the generator in the medium, means for receiving waves from the medium at a point removed from the generator,

and means for measuring the energy and amplif a@ tude of the received waves.

2. Apparatus for measuring the acoustical 1mpedance of a medium at a point, which comprises an elastic wave generator for supplying elastic wave energy to the medium, means for driving 65 the generator, means for measuring the energy supplied to drive the generator, and means for measuring the amplitude of the lelastic energy supplied to the medium.

3. Apparatus for measuring the acoustical imm pedance of a medium at a point, which comprises an elastic wave generator for supplying elastic wave energy to the medium, an oscillator operatively connected to the generator, means for measuring the electrical energy supplied by the m oscillator to the generator, and means for measuring the amplitude oi the elastic energy .supplied to the medium.

4. Apparatus for measuring the acoustical impedance of a medium surrounding a point, which comprises an elastic wave generator, including it a diaphragm coupled to the medium for supplyi ing elastlcwave energy to the medium, an oscillator operatively connected to the generator to actuate the diaphragm, means for measuring the electrical energy supplied by the oscillator to the 10 generator, and means for measuring the amplitude of vibration of the diaphragm.

5. The method for measuring the acoustical impedance of a medium surrounding a point, which comprises generating elastic energy in the me 1&5 dium, and-measuring lthe load on the generator due to this medium.

6. The method of measuringV the acoustical properties of a medium, which comprises actuating a vibratory member coupled to the medium, 'if and observing the driving force of the member and the effect oi the acoustical impedance of the medium on the displacement of this member.

7. The method of measuring the acoustical properties of a medium which comprises actuating a vibratory member coupled to the medium with a succession of continuous waves of given frequencies and successively measuring the energy actuating the .vibratory member, and the eect of the medium on the amplitude of the vibratory a@ member. l

8. The method of logging a well, which comprises generating elastic waves of given frequency at a source in the well whereby the waves traverse the ground adjoining the well, measuring the gg energy supplied to actuate the generator, receiv ing these waves at a receiver disposed in the well, measuring the energy and amplitude of the received waves, and varying the vertical positions of the source and receiver in the well whereby Q1@ variationsin the acoustical attenuation of the elastic waves due to variations in the ground are determined.

9. The method of logging a Well, which comprises generating elastic waves in the Well where- 4,5 by the waves traverse the ground adjoining the well, varying the vertical position of the generator in the well, measuring the energy supplied to drive the generator, and measuring the amplitude of the elastic waves in the ground adjoining the se well whereby variations in the acoustical properties of the ground are measured.

10. The method of logging a well, which comprises supplying electrical energy to a generator of elastic waves in the well whereby the ampli- 5g tude of the generator is adected by the acoustical impedance of the ground adjoining the generator, measuring the electrical energy supplied to the generator, measuring the amplitude of the generator, and varying the vertical position of the c@ generator in the well whereby the acoustical impedance of the ground at the vertical positions is determined.

1l. The method of logging a well, which comprises generating elastic energy at a source in g5 the well whereby the energy traverses the ground adjoining the well, 'varying the vertical position of the source in the well, and measuring the eect of the ground on the transmitted elastic energy adjoining the diierent vertical positions of the source whereby acoustical properties of the ground adjoining the source are measured.

12. The method of logging a well, which comprises generating elastic energy at a source in the well whereby the energy is transmitted to the ms ground adjoining the well, varying the vertical position of thgsource in the well, and measuring variations ixthe load on the generator due to variations in the ground adjoining the source whereby variations in the acoustical impedance of the ground are measured. l

13. The method of logging a well, which comprises actuating a vibratory member at a source in the well whereby the vibrations are transmitted to the ground adjoining the Well, varying the vertical position of the source in the well, and observingthe variations in the driving force of the member and the eiect of the acoustical impedance'of the ground on the displacement of come themember at the `various vertical positions of the source.

14. The method of logging a well, which comprisesactuating a vibratory member 4at a source in the we11 whereby vibrations are transmitted to the ground adjoining the well with a succession of waves of given frequencies, varying the vertical position of the source in the well, successively measuring variations in the energy required for actuating the vibratory member, and successively measuring the effect of the ground on the amplitude of the vibratory member.

WHITMAN D. MOUNCE. 

